Image sensor

ABSTRACT

An image sensor may include a photoelectric conversion element, a transfer transistor formed over the photoelectric conversion element, and a reset transistor formed over the photoelectric conversion element, formed substantially at the same level as the transfer transistor, and spaced apart from the transfer transistor by a gap, wherein the transfer transistor and the reset transistor are configured symmetrical to each other with respect to the gap.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to Korean Patent Application No.10-2016-0062759, filed on May 23, 2016, which is herein incorporated byreference in its entirety.

BACKGROUND

Exemplary embodiments of the present invention relate to a semiconductordevice fabrication technology, and more particularly, to an imagesensor.

An image sensor converts an optical image into an electrical signal.Recently, due to developments in the computer and communicationindustries, a demand for an image sensor with improved performance hasincreased for various devices, such as digital cameras, camcorders,Personal Communication System (PCS), game machines, security cameras,medical micro-cameras, and robots.

SUMMARY

Various embodiments are directed to an image sensor having improvedperformance.

In an embodiment, an image sensor may include a photoelectric conversionelement; a transfer transistor formed over the photoelectric conversionelement; and a reset transistor formed over the photoelectric conversionelement, formed substantially at the same level as the transfertransistor, and spaced apart from the transfer transistor by a gap,wherein the transfer transistor and the reset transistor are configuredsymmetrical to each other with respect to the gap

The transfer transistor may include: a first insulating layer formedover the photoelectric conversion element; a first conductive layerformed over the first insulating layer; a first open portion formedthrough the first insulating layer and the first conductive layer toexpose the photoelectric conversion element; a first channel layerformed in the first open portion, and contacting the photoelectricconversion element and the first conductive layer; and a first gateformed over the first channel layer. The reset transistor may include: asecond insulating layer formed over the photoelectric conversionelement; a second conductive layer formed over the second insulatinglayer; a second open portion formed through the second insulating layerand the second conductive layer to expose the photoelectric conversionelement; a second channel layer formed in the second open portion andcontacting the photoelectric conversion element and the secondconductive layer; and a second gate formed over the second channellayer. The first gate may include: a first gate insulating layer formedover the first channel layer; and a first gate electrode formed over thefire gate insulating layer and filling the first open portion. Thesecond gate may include: a second gate insulating layer formed over thesecond channel layer; and a second gate electrode formed over the secondgate insulating layer and filling the second open portion. The firstchannel layer may include undoped polysilicon or P type polysilicon, andwherein the second channel layer may include undoped polysilicon or Ptype polysilicon. The first channel layer may include N typepolysilicon, and wherein the second channel layer may include N typepolysilicon.

The photoelectric conversion element located under the transfertransistor may have the same size as the photoelectric conversionelement located under the reset transistor. The gap may be formed overthe photoelectric conversion element and may have a line shape whichcomprises a central point of the photoelectric conversion element andextends in a row direction, column direction or diagonal direction.

In an embodiment, an image sensor may include a photoelectric conversionelement; an insulating layer formed over the photoelectric conversionelement; a first conductive layer formed over the insulating layer; asecond conductive layer formed over the insulating layer, formed at thesame level as the first conductive layer, spaced apart from the firstconductive layer by a gap, and configured symmetrical to the firstconductive layer with respect to the gap; a first open portion formedthrough the first conductive layer and the insulating layer to exposethe photoelectric conversion element; a second open portion formedthrough the second conductive layer and the insulating layer to exposethe photoelectric conversion element, and configured symmetrical to thefirst open portion with respect to the gap; a first channel layer formedin the first open portion and contacting the photoelectric conversionelement and the first conductive layer; a second channel layer formed inthe second open portion, contacting the photoelectric conversion elementand the second conductive layer, and configured symmetrical to the firstchannel layer with respect to the gap; a transfer gate formed over thefirst channel layer and filling the first open portion; and a reset gateformed over the second channel layer, filling the second open portion,and configured symmetrical to the transfer gate with respect to the gap.

The transfer gate may include: a first gate insulating layer formed overthe first channel layer; and a first gate electrode formed over thefirst gate insulating layer and filling the first open portions Thereset gate may include: a second gate insulating layer formed over thesecond channel layer; and a second gate electrode formed over the secondgate insulating layer and filling the second open portion. The firstconductive layer and the second conductive layer may be formed of thesame material as each other, wherein the first channel layer and thesecond channel layer may be formed of the same material as each other,and wherein the transfer gate and the reset gate may be formed of thesame material as each other. Each of the first channel layer and thesecond channel layer may include undoped polysilicon or P typepolysilicon. Each of the first channel layer and the second channellayer may include N type polysilicon. The photoelectric conversionelement covered by the first conductive layer may have the same size asthe photoelectric conversion element covered by the second conductivelayer. The gap may be formed over the photoelectric conversion elementand may have a line shape which comprises a central point of thephotoelectric conversion element and extends in a row direction, columndirection or diagonal direction.

In an embodiment, an image sensor may include a first node receiving afirst voltage; a second node receiving a second voltage; a third nodecoupled between the first node and the second node; a photoelectricconversion element coupled between the third node and the second node; areset transistor coupled between the third node and the first node; atransfer transistor between the third node and a fourth node; and afloating diffusion coupled between the second node and the fourth node.The image sensor may further include a drive transistor including afirst gate, a first source/drain, and a second source/drain, wherein thefirst gate is coupled to the fourth node, wherein the first source/drainis coupled to the first node, wherein the second source/drain is coupledto a column line; and a selection transistor including a second gate, athird source/drain, and a fourth source/drain, wherein the second gateis coupled to a row line, wherein the third source/drain is coupled tothe second source/drain of the drive transistor, wherein the fourthsource/drain is coupled to the column line.

The transfer transistor and the reset transistor may be symmetricalstructure and may have substantially the same structure as each other.The first voltage may have a higher level than the second voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating an image sensor inaccordance with an embodiment of the present invention.

FIG. 2 is a plane view showing part of the pixel array of an imagesensor in accordance with an embodiment of the present invention.

FIG. 3 is a cross-sectional view of the image sensor taken along theline A-A′ of FIG. 2 in accordance with an embodiment of the presentinvention.

FIG. 4 is an equivalent circuit diagram of a unit pixel of an imagesensor in accordance with an embodiment of the present invention.

FIG. 5 is a schematic diagram illustrating operational timing of animage sensor in accordance with an embodiment of the present invention.

FIGS. 6a and 6b are plane views illustrating part of a pixel array of animage sensor in accordance with a modified example.

FIG. 7 is a diagram schematically illustrating an electronic deviceincluding an image sensor in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

Various embodiments will be described below in more detail withreference to the accompanying drawings. The present invention may,however, be embodied in different forms and should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art. Throughout the disclosure, like reference numerals refer tolike parts throughout the various figures and embodiments of the presentinvention.

The drawings are not necessarily to scale and in some instances,proportions may have been exaggerated to clearly illustrate features ofthe embodiments. When a first layer is referred to as being “on” asecond layer or “on” a substrate, it not only refers to a case in whichthe first layer is formed directly on the second layer or the substratebut also a case in which a third layer exists between the first layerand the second layer or the substrate.

Embodiments of the present invention to be described later relate to animage sensor having improved performance and a method for driving thesame. In this case, the image sensor having improved performance maymean an image sensor capable of providing an image of a high pixel/highquality. In order to provide an image of a high pixel, there is a needfor an image sensor in which a plurality of unit pixels have beenintegrated within a limited area. Furthermore, to provide an image ofhigh quality, there is a need for an image sensor capable of preventingthe deterioration of characteristics attributable to the high degree ofintegration of a plurality of unit pixels. Accordingly, an image sensorin accordance with an embodiment may include a plurality of unit pixels,each including a transfer transistor having a vertical channel and areset transistor having a vertical channel. The transfer transistor andthe reset transistor may have a symmetrical structure. Furthermore, thetransfer transistor and the reset transistor may have theirphotoelectric conversion elements vertically overlapped.

FIG. 1 is a block diagram schematically illustrating an image sensor inaccordance with an embodiment of the present invention. As illustratedin FIG. 1, the image sensor may include a pixel array 100, a correlateddouble sampling (CDS) unit 120, an analog-digital converter (ADC) 130, abuffer 140, a row driver 150, a timing generator 160, a control register170 and a ramp signal generator 180. The pixel array 100 may include aplurality of unit pixels 110 arranged in a matrix.

The timing generator 160 may generate one or more control signals forcontrolling the row driver 150, the CDS unit 120, the ADC 130, and theramp signal generator 180. The control register 170 may generate one ormore control signals for controlling the ramp signal generator 180, thetiming generator 160 and the buffer 140.

The row driver 150 may drive the pixel array 100 row by row. Forexample, the row driver 150 may generate a select signal for selectingany one row line of a plurality of row lines. Each of the unit pixels110 may sense incident light and output an image reset signal and animage signal to the CDS unit 120 through a column line. The CDS unit 120may perform sampling in response to the image reset signal and the imagesignal.

The ADC 130 may compare a ramp signal outputted from the ramp signalgenerator 180 with a sampling signal outputted from the CDS unit 120,and output a comparison signal. According to a clock signal providedfrom the timing generator 160, the ADC 130 may count the leveltransition time of the comparison signal, and output the count value tothe buffer 140. The ramp signal generator 180 may operate under controlof the timing generator 160.

The buffer 140 may store a plurality of digital signals outputted fromthe ADC 130 and then sense and amplify the digital signals. Thus, thebuffer 140 may include a memory (not illustrated) and a sense amplifier(not illustrated). The memory may store count values. The count valuesmay represent signals outputted from the plurality of unit pixels 110.The sense amplifier may sense and amplify the count values outputtedfrom the memory.

In this case, in order to provide an image of a high pixel, the numberof unit pixels 110 integrated within the, pixel array 100 may need to beincreased. That is, more unit pixels 110 need to be disposed within alimited area. To this end, the physical size of the unit pixel 110 needsto be reduced. However, the image sensor operates based on a pixelsignal generated from each of the unit pixels 110 in response toincident light. If the physical size of the unit pixel 110 is reduced,characteristics are inevitably deteriorated since the number of photonsreceived by the unit pixel 100 is reduced.

Furthermore, each of the unit pixels 110 may include a photoelectricconversion element configured to generate photocharges in response toincident light and an output circuit configured to output an imagesignal in response to the generated photocharges. The output circuit mayinclude a plurality of pixel transistors. More specifically, theplurality of pixel transistors may include a transfer transistor Tx, areset transistor Rx, a drive transistor Dx and a selection transistorSx. For reference, the drive transistor Dai is also called a sourcefollower transistor.

In this case, characteristics may be deteriorated since the plurality ofpixel transistors of each of the unit pixels 110 has different forms andsizes. More specifically, the transfer transistor Tx configured totransfer photocharges generated by the photoelectric conversion elementto a floating diffusion FD and the reset transistor Rx configured toreset the floating diffusion FD have different forms and sizes.Accordingly, the amount of charges introduced into the floatingdiffusion FD when the reset transistor Rx switches and the amount ofcharges introduced into the floating diffusion FD when the transfertransistor Tx switches are different. This may deteriorate quality of animage since a noise is increased in an image signal and image resetsignal generated by each unit pixel 110.

Accordingly, an image sensor capable of facilitating the high degree ofintegration and preventing the deterioration of characteristicsattributable to an increase of the degree of integration in accordancewith an embodiment of the present invention is described below in detailwith reference to related drawings.

FIG. 2 is a plane view showing part of a pixel array of an image sensorin accordance with an embodiment of the present invention. FIG. 3 is across-sectional view of the image sensor taken along the line A-A′ ofFIG. 2 in accordance with an embodiment of the present invention. FIG. 4is an equivalent circuit diagram of a unit pixel of an image sensor inaccordance with an embodiment of the present invention. Furthermore,FIG. 5 is a schematic diagram illustrating operating timing of an imagesensor in accordance with an embodiment of the present invention.

As shown in FIGS. 2 to 5, the image sensor in accordance with anembodiment may include a photoelectric conversion element PD, thetransfer transistor Tx formed on the photoelectric conversion elementPD, and the reset transistor Rx formed on the photoelectric conversionelement PD and configured to be adjacent to the transfer transistor Txwith a gap 232 Interposed therebetween. In this case, the transfertransistor Tx and the reset transistor Rx may have a symmetricalstructure based on the gap 232. The image sensor in accordance with anembodiment is described in more detail below.

First, a plane shape is described with reference to FIG. 2. In the imagesensor in accordance with an embodiment, each of the unit pixels 110 mayinclude a first region 110-1 and a second region 110-2. The first region110-1 may include the photoelectric conversion element PD, the transfertransistor Tx, and the reset transistor Rx. The second region 110-2 mayinclude an active region 208, the drive transistor Dx, and the selectiontransistor Sx. The first region 110-1 and the second region 110-2 may beseparated from each other by an isolation structure 202.

In the first region 110-1, the transfer transistor Tx and the resettransistor Rx may be configured to overlap the photoelectric conversionelement PD. The transfer transistor Tx and the reset transistor Rx mayhave a symmetrical form based on the gap 232 over the photoelectricconversion element D. More specifically, the transfer transistor Tx andthe reset transistor Rx may have a left and right symmetrical form basedon the gap 232. In this case, the gap 232 may have a line shape, whichextends across the photoelectric conversion element PD in a columndirection. In other words, the gap 232 may have a line shape whichincludes the central point of the photoelectric conversion element,extends in a column direction, and intersects the photoelectricconversion element PD. The photoelectric conversion element PD placed onone side of the gap 232 may have the same size as the photoelectricconversion element. PD placed on the other side of the gap 232. Forreference, although not shown, the gap 232 may also extend in a rowdirection across the photoelectric conversion element PD.

The transfer transistor Tx may be electrically coupled to the floatingdiffusion FD and the drive transistor Dx through a first contact C1.Furthermore, the reset transistor Rx may be coupled to a first node 310through a second contact C2. A first voltage may be supplied to thefirst node 310. In this case, the first voltage may be a power sourcevoltage VDD or a voltage having a higher level than the power sourcevoltage VDD. For reference, the first contact C1 and the second contactC2 may be symmetrically disposed with respect to the gap 323, but may beasymmetrically disposed in another embodiment.

In the second region 110-2, the drive transistor Dx has a gate coupledto the floating diffusion FD, and may generate an output voltagecorresponding to the amount of charges stored in the floating diffusionFD, that is an image signal and an image reset signal. The selectiontransistor Sx has a gate coupled to a row line and may output an outputvoltage generated by the drive transistor Dx to a column line inresponse to a selection signal SEL.

The drive transistor Dx and the selection transistor Sx may be disposedto share a junction region, that is, a source region or a drain region.The gate of the drive transistor Dx may be greater size than the gate ofthe selection transistor Sx. The drive transistor Dx may be coupled tothe first node 310, to which the first voltage is supplied, through athird contact C3 formed on one side of the active region 208.Furthermore an image signal and an image reset signal generated by theunit pixel 110 may be output to the column line through a fourth contactC4 formed on the other side of the active region 208.

A cross-sectional shape is described below with reference to FIG. 3. Theimage sensor in accordance with an embodiment may include a substrate200. The photoelectric conversion element PD and the isolation structure202 are formed in the substrate 200. The photoelectric conversionelement PD and the active region 208 are formed in the substrate 200,separated from each other, and adjacent to each other.

The substrate 200 may include a semiconductor substrate. Thesemiconductor substrate may have a single crystal state and may includea silicon-containing material. That is, the substrate 200 may include asingle crystal silicon-containing material. Furthermore, the substrate200 may be a substrate that has been thinned through a thinning process.For example, the substrate 200 may be a bulk silicon substrate that hasbeen thinned through a thinning process.

The isolation structure 202 may include a Shallow Trench Isolation(STI), a Deep Trench Isolation (DTI) or a potential barrier. Thepotential barrier may include an impurity region formed by dopingimpurities into the substrate 200. For example, the potential barriermay be a P type impurity region formed by doping boron, that is, P typeimpurities, into the substrate 200. The isolation structure 202 mayinclude any one of the STI, the DTI, and the potential barrier or mayinclude a combination thereof. For example, the isolation structure 202surrounding the photoelectric conversion element PD may be the DTI ormay have a structure in which the DTI and the potential barrier havebeen coupled. Furthermore, the isolation structure 202 configured toseparate the photoelectric conversion element PD and the active region208 may be the STI or may have a structure in which the STI and thepotential barrier have been combined.

The photoelectric conversion element PD may include an organic orinorganic photodiode. For example, the photoelectric conversion elementPD may include a first impurity region 204 and a second impurity region206 which are formed in the substrate 200, have complementary conductivetypes, and have been vertically stacked. The first impurity region 204may have a smaller thickness than the second impurity region 206. Thefirst impurity region 204 and the second impurity region 206 may havebeen formed by implanting impurities into the substrate 200. Morespecifically, the first impurity region 204 may be a P type impurityregion, and the second impurity region 206 may be an N type impurityregion.

The image sensor in accordance with an embodiment may include thetransfer transistor Tx and the reset transistor Rx which are formed onthe photoelectric conversion element PD and configured to verticallyoverlap the photoelectric conversion element PD. The transfer transistorTx and the reset transistor Rx may have the gap 232 over thephotoelectric conversion element PD, may be adjacent to each other, andmay have a symmetrical form based on the gap 232. A fill factor of thephotoelectric conversion element PD can be maximized and the degree ofintegration of the unit pixels 110 can be easily improved since each ofthe transfer transistor Tx and the reset transistor Rx verticallyoverlaps the photoelectric conversion element PD as described above.Furthermore, image quality can be improved since the transfer transistorTx and the reset transistor Rx are symmetrical to each other instructure, in shape, and in size.

The transfer transistor Tx may include an insulating layer 210 formed onthe photoelectric conversion element PD, a first conductive layer 212formed on the insulating layer 210, a first open portion 214 configuredto expose the photoelectric conversion element PD through the insulatinglayer 210 and the first conductive layer 212, a first channel layer 216formed along a surface of the first open portion 214 and coupled to thefirst conductive layer 212 and the photoelectric conversion element PD,and a transfer gate 220 formed on the first channel layer 216 andfilling in the first open portion 214. The transfer gate 220 may includea first gate insulating layer 218 formed on the first channel layer 216and a first gate electrode 219 formed on the first gate insulating layer218 and filling in the first open portion 21.4

The first conductive layer 212 and the photoelectric conversion elementPD may function as the junction region, that is, the source region andthe drain region, of the transfer transistor Tx. More specifically, thefirst conductive layer 212 may function as the floating diffusion FD.The first channel layer 216 may function to electrically couple thephotoelectric conversion element PD and the first conductive layer 212in response to a transmission signal TRF applied to the transfer gate220.

In an embodiment, the first channel layer 216 is formed on the firstconductive layer 212 in addition to the bottom and side of the firstopen portion 214. However, the first channel layer 216 may have anyshape so long as the first channel layer 216 is in contact with thephotoelectric conversion element PD and the first conductive layer 212.For example, the first channel layer 216 may have a cylinder shape andbe formed on the bottom and side of the first open portion 214.

The reset transistor Rx may include the insulating layer 210 formed onthe photoelectric conversion element PD, a second conductive layer 242formed on the insulating layer 210, a second open portion 244 configuredto expose the photoelectric conversion element PD through the insulatinglayer 210 and the second conductive layer 242, a second channel layer246 formed along a surface of the second open portion 244 and coupled tothe second conductive layer 242 and the photoelectric conversion elementPD, and a reset gate 250 formed on the second channel layer 246 andconfigured to gap-fill the second open portion 244. The reset gate 250may include a second gate insulating layer 248 formed on the secondchannel layer 246 and a second gate electrode 249 formed on the secondgate insulating layer 248 and configured to gap-fill at least the secondopen portion 244.

The second conductive layer 242 and the photoelectric conversion elementPD may function as junction regions of the reset transistor Rx. Thesecond channel layer 246 may electrically couple the photoelectricconversion element PD and the second conductive layer 242 in response toa reset signal RST applied to the reset gate 250. In FIG. 3, the secondchannel layer 246 is formed on the second conductive layer 242 inaddition to the bottom and side of the second open portion 244. However,the second channel layer 246 may have any shape so long as the secondchannel layer 246 is in contact with the photoelectric conversionelement PD and the second conductive layer 242. For example, the secondchannel layer 246 may have a cylinder shape and be formed on the bottomand side of the second open portion 244.

The insulating layer 210 may have a flat panel form and may have agreater area than the photoelectric conversion element PD. The channellength of the transfer transistor Tx and the reset transistor Rx may becontrolled based on the thickness of the insulating layer 210. Theinsulating layer 210 may include oxide, nitride, oxynitride, and acombination thereof.

The first conductive layer 212 and the second conductive layer 242 mayhave a symmetrical structure with respect to the gap 232. Morespecifically, the first conductive layer 212 and the second conductivelayer 242 may have a left and right symmetrical form based on the gap232. Each of the first conductive layer 212 and the second conductivelayer 242 may include a semiconductor material or a metallic material.For example, each, of the first conductive layer 212 and the secondconductive layer 242 may include a silicon-containing material. Thesilicon-containing material may be polysilicon. N type impurities mayhave been doped into the polysilicon. The first conductive layer 212 andthe second conductive layer 242 may have been formed at the same timeand include the same material as each other.

The first open portion 214 and the second open portion 244 may have asymmetrical structure with respect to the gap 232. More specifically,the first open portion 214 and the second open portion 244 may have aleft and right symmetrical form with respect to the gap 232. In thefirst open portion 214 and the second open portion 244, a channel layerand a gate will be formed. The first open portion 214 and the secondopen portion 244 may have various plane shapes. For example, the planeshape of each of the first open portion 214 and the second open portion244 may be a polygon, circle, or oval. Each of the first open portion214 and the second open portion 244 may have vertical sidewalls or mayhave inclined sidewalls of which an internal line width is reducedtoward the photoelectric conversion element PD.

The first open portion 214 and the second open portion 244 may be formedat the same time and have the same shape as each other. In anembodiment, the number of each of the first open portions 214 and thesecond open portions 244 may be one as shown in FIG. 3, but the numberof each of the first open portions 214 and the second open portions 244is not limited thereto. That is, the number of each of the first openportions 214 and the second open portions 244 may be more than one. Asthe number of each of the first open portions 214 and the second openportions 244 increases, the channel width of the transfer transistor Txand the reset transistor Rx may be increased.

The first channel layer 216 and the second channel layer 246 may have asymmetrical structure with respect to the gap 232. More specifically,the first channel layer 216 and the second channel layer 246 may have aleft and right symmetrical form with respect to the gap 232. The firstchannel layer 216 and the second channel layer 246 may have been formedat the same time and have the same material as each other. Each of thefirst channel layer 216 and the second channel layer 246 may include asilicon-containing material. For example, each of the first channellayer 216 and the second channel layer 246 may include polysilicon. Morespecifically, each of the first channel layer 216 and the second channellayer 246 may include undoped polysilicon, P type polysilicon, or N typepolysilicon.

When the first channel layer 216 and the second channel layer 246include undoped polysilicon or P type polysilicon, the transfertransistor Tx and the reset transistor Rx may operate in an enhancementmode in which a channel maintains an inactive state in an off state. Incontrast, when the first channel layer 216 and the second channel layer246 include N type polysilicon, the transfer transistor Tx and the resetns star Rx may operate in a depletion mode in which a channel maintainsan active state in the off state. When the transfer transistor Tx andthe reset transistor Rx operate in a depletion mode, image quality canbe improved more effectively in a low illuminance environment.

The transfer gate 220 and the reset gate 250 may have a symmetricalstructure with respect to the gap 232. More specifically, the transfergate 220 and the reset gate 250 may have a left and right symmetricalform with respect to the gap 232. The transfer gate 220 and the resetgate 250 may have been formed at the same time and include the samematerial as each other. More specifically, the first gate insulatinglayer 218 and the second gate insulating layer 248 may have been formedat the same time and include the same material as each other. Each ofthe first gate insulating layer 218 and the second gate insulating layer248 may include oxide, nitride, oxynitride, or a combination thereof.The first gate electrode 219 and the second gate electrode 249 may havebeen formed at the same time and include the same material as eachother, Each of the first gate electrode 219 and the second gateelectrode 249 may include a semiconductor material or a metallicmaterial.

The image sensor in accordance with an embodiment may include aninterlayer dielectric layer 230 formed over the substrate 200 andconfigured to cover the transfer transistor Tx and the reset transistorRx, a color separation element 260 formed on an incident surface throughwhich incident light is incident on the photoelectric conversion elementPD, and a light focusing element 270 formed on the color separationelement 260.

The interlayer dielectric layer 230 may include oxide, nitride,oxynitride, or a combination thereof. The first contact C1 may becoupled to the first conductive layer 212 through the interlayerdielectric layer 230, and the second contact C2 may be coupled to thesecond conductive layer 242 through the interlayer dielectric layer 230.The color separation element 260 may include a color filter. The colorfilter may include a red filter, a green filter, a blue filter, a cyanfilter, a yellow filter, a magenta filter, a white filter, a blackfilter, an IR cutoff filter, etc. The light focusing element 270 mayinclude a digital lens or a hemispherical lens.

Referring to FIG. 4, the equivalent circuit diagram of the unit pixel110 of the image sensor in accordance with an embodiment may include athird node 330 coupled between a first node 310 set as a first voltageand a second node 320 set as a second voltage. The first voltage mayhave a higher level than the second voltage. For example, the firstvoltage may be a power source voltage VDD or a voltage having a higherlevel than the power source voltage VDD. The second voltage may be aground voltage VSS. The reset transistor Rx may be coupled between thefirst node 310 and the third node 330, and the photoelectric conversionelement PD may be coupled between the third node 330 and the second node320. The reset signal RST may be applied to the gate of the resettransistor Rx.

One side of the transfer transistor Tx may be coupled the third node330, and the other side thereof may be coupled to the gate of the drivetransistor Dx. A fourth node 340 may be coupled between the other sideof the transfer transistor Tx and the gate of the drive transistor Dx.The floating diffusion FD may be coupled between the second node 320 andthe fourth node 340. That is, the fourth node 340 may function as afloating diffusion node. The transmission signal TRF may be applied tothe gate of the transfer transistor Tx.

One side of the drive transistor Dx may be coupled to the first node 10and the other side thereof may be coupled to the selection transistorSx. The gate of the selection transistor Sx may be coupled the row line,and one side and the other side of the selection transistor Sx may becoupled to the column line and the drive transistor Dx, respectively.The selection signal SEL may be applied to the selection transistor Sxthrough the row line.

A method for driving the image sensor in accordance with an embodimentis described below with reference to FIG. 5 During a first section T1,the reset transistor Rx and the transfer transistor Tx are activated byapplying the reset signal RST and the transmission signal TRF, therebyresetting the fourth node 340, that is, the floating diffusion FD. Next,the reset transistor Rx and the transfer transistor Tx are turned off.During a second section T2, photocharges are generated by radiatingincident light to the photoelectric conversion element PD. The secondsection T2 is also called an integration time.

Next, during a third section T3, the selection transistor Sx and thetransfer transistor Tx are activated by applying the selection signalSEL and the transmission signal TRF, respectively. When the transfertransistor Tx is activated, the photocharges generated from thephotoelectric conversion element PD during the second section T2 aretransferred and stored in the floating diffusion FD. The drivetransistor Dx generates an output voltage corresponding to the amount ofphotocharges stored in the floating diffusion FD that is, an imagesignal. The generated image signal may be output to the column linethrough the selection transistor Sx.

Next, the selection transistor Sx and the transfer transistor Tx aremaintained activated during a fourth section T4. The reset transistor Rxis also activated by applying the reset signal RST. When the resettransistor Rx is activated and all of the photocharges stored in thefloating diffusion FD have exited, the drive transistor Dx generates anoutput voltage, corresponding to the state in which the floatingdiffusion FD has been reset, that is, an image reset signal, and mayoutput the generated image reset signal to the column line through theselection transistor Sx.

As described above, the image sensor in accordance with an embodimentcan effectively remove the distortion of analog data which is generatedwhen a signal applied to the gates of the reset transistor Rx and thetransfer transistor Tx is changed. Accordingly, data according tophotocharges generated by the photoelectric conversion element PD can beoutput without distortion, and thus a high quality of image can beprovided.

In the aforementioned embodiment, the gap 232 between the transfertransistor Tx and the reset transistor Rx have a symmetrical structureto each other, are provided on the photoelectric conversion element PD,and are illustrated as having a line shape extending in the rowdirection or the column direction. In a modified example, the gap 232may have a line shape extending in a diagonal direction between the rowdirection and the column direction. This is described below withreference to FIGS. 6a and 6 b.

As shown in FIGS. 6a and 6b , the image sensor in accordance with amodified example may include the photoelectric conversion element PD,the transfer transistor Tx formed on the photoelectric conversionelement PD, and the reset transistor Rx formed on the photoelectricconversion element PD and adjacent to the transfer transistor Tx withthe gap 232 interposed therebetween. In this case, the transfertransistor Tx and the reset transistor Rx may have a symmetricalstructure based on the gap 232. More specifically, the transfertransistor Tx may be congruent or similar to the reset transistor Rxwhen rotated 180° with respect to the gap 232.

As shown in FIG. 6a , in each of the plurality of unit pixels 110, thegap 232 may have a line shape and extend in a diagonal direction. In theplurality of unit pixels 110, all of the gaps 232 may have line shapesand extend in the same direction. In this case, the gap 232 may extendacross the middle of the photoelectric conversion element PD in thediagonal direction. The gap 232 may include the central point of thephotoelectric conversion element PD. The area of the photoelectricconversion element PD placed on one side of the gap 232 and the area ofthe photoelectric conversion element PD placed on the other side of thegap 232 may be the same as each other in size.

As shown in FIG. 6 b, in each of the plurality of unit pixels 100 thegap 232 has a line shape and extends in a diagonal direction. However,in the unit pixels 100, the directions in which the gaps 232 have beenextended may be different from FIG. 6a . For example a plurality of thegaps 232 adjacent to each other in the pixel array may be disposed tohave a diamond shape. Although not shown this shape facilitates thedesign of wires coupled to the first contact C1 and the second contactC2 and s coupled to the transfer gate 220 and the reset gate 250.Furthermore, the shape shown in FIG. 6b may be easily applied to animage sensor having a pixel sharing structure.

The image sensor in accordance with an embodiment of the presentinvention may be used in various electronic devices or systems.Hereafter, a camera including an image sensor in accordance with anembodiment of the present invention will be described with reference toFIG. 7.

FIG. 7 is a diagram schematically illustrating an electronic deviceincluding an image sensor in accordance with an embodiment of thepresent invention. Referring to FIG. 7, the electronic device includingthe image sensor in accordance with an embodiment of the presentinvention may be a camera capable of taking a still age or a movingimage. The electronic device may include an optical system or opticallens 910, a shutter unit 911, a driving unit 913 for controlling/drivingthe image sensor 900 and the shutter unit 911, and a signal processingunit 912.

The optical system 910 may guide image light from an object to the pixelarray 100 of the image sensor 900. The optical system 910 may include aplurality of optical lenses. The shutter unit 911 may control a lightirradiation period and a light shield period for the image sensor 900.The driving unit 913 may control a transmission operation of the imagesensor 900 and a shutter operation of the shutter unit 911. The signalprocessing unit 912 may process signals outputted from the Image sensor900 in various manners. The processed image signals Dout may be storedin a storage medium such as a memory or outputted to a monitor or thelike.

The present embodiments may increase a degree of integration since thetransfer transistor and the reset transistor are configured to overlapthe photoelectric conversion element.

Furthermore, an image of high quality can be provided since the transfertransistor and the reset transistor are symmetrical to each other instructure.

Although various embodiments have been described for illustrativepurposes, it will be apparent to those skilled in the art that variouschanges and modifications may be made without departing from the spiritand scope of the invention as defined in the following claims.

1. An image sensor, comprising: a photoelectric conversion element; atransfer transistor formed over the photoelectric conversion element;and a reset transistor formed over the photoelectric conversion element,formed substantially at the same level as the transfer transistor, andspaced apart from the transfer transistor by a gap, wherein the transfertransistor and the reset transistor are configured symmetrical to eachother with respect to the gap.
 2. The image sensor of claim 1, whereinthe transfer transistor comprises: a first insulating layer formed overthe photoelectric conversion element; a first conductive layer formedover the first insulating layer; a first open portion formed through thefirst insulating layer and the first conductive layer to expose thephotoelectric conversion element; a first channel layer formed in thefirst open portion, and contacting the photoelectric conversion elementand the first conductive layer; and a first gate formed over the firstchannel layer, and wherein the reset transistor comprises: a secondinsulating layer formed over the photoelectric conversion element; asecond conductive layer formed over the second insulating layer; asecond open portion formed through the second insulating layer and thesecond conductive layer to expose the photoelectric conversion element;a second channel layer formed in the second open portion and contactingthe photoelectric conversion element and the second conductive layer;and a second gate formed over the second channel layer.
 3. The imagesensor of claim 2, wherein the first gate comprises: a first gateinsulating layer formed over the first channel layer; and a first gateelectrode formed over the first gate insulating layer and filling thefirst open portion, and wherein the second gate comprises: a second gateinsulating layer formed over the second channel layer; and a second gateelectrode formed over the second gate insulating layer and filling thesecond open portion.
 4. The image sensor of claim 2, wherein the firstchannel layer comprises undoped polysilicon or P type polysilicon, andwherein the second channel layer comprises undoped polysilicon or P typepolysilicon.
 5. The image sensor of claim 2, wherein the first channellayer comprises N type polysilicon, and wherein the second channel layercomprises N type polysilicon.
 6. The image sensor of claim 1, whereinthe photoelectric conversion element located under the transfertransistor has the same size as the photoelectric conversion elementlocated under the reset transistor.
 7. The image sensor of claim 6,wherein the gap is formed over the photoelectric conversion element andhas a line shape which comprises a central point of the photoelectricconversion element and extends in a row direction, or column direction.8. An image sensor, comprising: a photoelectric conversion element; aninsulating layer formed over the photoelectric conversion element; afirst conductive layer formed over the insulating layer; a secondconductive layer formed over the insulating layer, formed at the samelevel as the first conductive layer, spaced apart from the firstconductive layer by a gap, and configured symmetrical to the firstconductive layer with respect to the gap; a first open portion formedthrough the first conductive layer and the insulating layer to exposethe photoelectric conversion element; a second open portion formedthrough the second conductive layer and the insulating layer to exposethe photoelectric conversion element, and configured symmetrical to thefirst open portion with respect to the gap; a first channel layer formedin the first open portion and contacting the photoelectric conversionelement and the first conductive layer; a second channel layer formed inthe second open portion, contacting the photoelectric conversion elementand the second conductive layer, and configured symmetrical to the firstchannel layer with respect to the gap; a transfer gate formed over thefirst channel layer and filling the first open portion; and a reset gateformed over the second channel layer, filling the second open portion,and configured symmetrical to the transfer gate with respect to the gap.9. The image sensor of claim 8, wherein the transfer gate comprises: afirst gate insulating layer formed over the first channel layer; and afirst gate electrode formed over the first gate insulating layer andfilling the first open portion, and wherein the reset gate comprises: asecond gate insulating layer formed over the second channel layer; and asecond gate electrode formed over the second gate insulating layer andfilling the second open portion.
 10. The image sensor of claim 8,wherein the first conductive layer and the second conductive layer areformed of the same material as each other, wherein the first channellayer and the second channel layer are formed of the same material aseach other, and wherein the transfer gate and the reset gate are formedof the same material as each other.
 11. The image sensor of claim 8,wherein each of the first channel layer and the second channel layerincludes undoped polysilicon or P type polysilicon.
 12. The image sensorof claim 8, wherein each of the first channel layer and the secondchannel layer includes N type polysilicon.
 13. The image sensor of claim8, wherein the photoelectric conversion element covered by the firstconductive layer has the same size as the photoelectric conversionelement covered by the second conductive layer.
 14. The image sensor ofclaim 13, wherein the gap is formed over the photoelectric conversionelement and has a line shape which comprises a central point of thephotoelectric conversion element and extends in a row direction, columndirection or diagonal direction.
 15. An image sensor, comprising: afirst node receiving a first voltage; a second node receiving a secondvoltage; a third node coupled between the first node and the secondnode; a photoelectric conversion element coupled between the third nodeand the second node; a reset transistor coupled between the third nodeand the first node; a transfer transistor between the third node and afourth node; and a floating diffusion coupled between the second nodeand the fourth node.
 16. The image sensor of claim 15, furthercomprising: a drive transistor including a first gate, a firstsource/drain, and a second source/drain, wherein the first gate iscoupled to the fourth node, wherein the first source/drain is coupled tothe first node, wherein the second source/drain is coupled to a columnline; and a selection transistor including a second gate, a thirdsource/drain, and a fourth source/drain, wherein the second gate iscoupled to a row line, wherein the third source/drain is coupled to thesecond source/drain of the drive transistor, wherein the fourthsource/drain is coupled to the column line.
 17. The image sensor ofclaim 15, wherein the transfer transistor and the reset transistor aresymmetrical structure and have substantially the same structure as eachother.
 18. The image sensor of claim 15, wherein the first voltage has ahigher level than the second voltage.
 19. The image sensor of claim 6,wherein the gap is formed over the photoelectric conversion element andhas a line shape which comprises a central point of the photoelectricconversion element and extends in a diagonal direction.
 20. The imagesensor of claim 13, wherein the gap is formed over the photoelectricconversion element and has a line shape which comprises a central pointof the photoelectric conversion element and extends in a diagonaldirection.